1. Field of the Invention
This invention relates to a liquid crystal display device and a method for manufacturing a liquid crystal display device, particularly a liquid crystal display device of an active matrix type using thin film transistors, or the like.
2. Description of the Prior Arts
In a liquid crystal display device of an active matrix type, non-linear devices (switching devices) are disposed in such a manner as to correspond to a plurality of pixel electrodes arranged in matrix, respectively. The liquid crystal in each pixel is always driven, in principle, i.e., it has a duty ratio of 1.0. In comparison with a so-called "simple matrix type" which employs a time division driving system, therefore, the active system has better contrast and has become an indispensable technique particularly in a color liquid crystal display device. A typical example of the switching devices is a thin film transistor (TFT).
FIG. 17 is a sectional view showing part of a conventional active matrix type liquid crystal display device. In this liquid crystal display device, a gate electrode GT is disposed on a transparent glass substrate SUB1, an insulating film GI used as a gate insulating film is disposed on the gate electrode GT, an i-type semiconductor layer AS is disposed on the insulating film GI, a video signal line DL is disposed on the insulating film GI, a source electrode SD11 and a drain electrode SD21 are disposed on both sides of the i-type semiconductor layer AS and a transparent pixel electrode ITO11 is disposed and connected with the source electrode SD11.
FIG. 18 is a sectional view showing part of another conventional active matrix type liquid crystal display device, such as described in the Proceedings of the SID, Vol. 31/1, 1990, pp 13-17. In this liquid crystal display device, a gate electrode GT is disposed on a transparent glass substrate SUB1, an insulating film GI1 consisting of a silicon nitride film is disposed on the gate electrode GT, a transparent pixel electrode ITO12 is disposed on the insulating film GI1, an insulating film GI2 consisting of a silicon nitride film is disposed on the transparent pixel electrode ITO12, an i-type semiconductor layer AS is disposed on the insulating film GI12, a video signal line DL is disposed on the insulating film GI12, a source electrode SD11 and a drain electrode SD21 are disposed on both sides of the i-type semiconductor layer AS, and the source electrode SD11 and the transparent pixel electrode ITO12 are connected through a through hole bored in the insulating film GI2.
In the liquid crystal display device shown in FIG. 17, the video signal line DL and the transparent pixel electrode ITO11 are formed on the same plane. Accordingly, the video signal line DL and the transparent pixel electrode ITO11 adjacent to each other undergo short-circuit and invite point defects and deterioration of display performance.
In the liquid crystal display device shown in FIG. 18, the video signal line DL and the transparent pixel electrode ITO12 are not formed on the same plane. Accordingly, the video signal line DL and the transparent pixel electrode ITO12 hardly short-circuit. However, two layers of insulating films GI1 and GI2 are disposed and if these insulating films GI1, GI2 are formed by a plasma CVD process, generation of dust due to the plasma reaction is extremely great inside a plasma CVD apparatus, so that defects are likely to occur in the insulating films GI1, GI2. For example, short circuits occur between the gate electrode GT and the source and drain electrodes SD11, SD21. Therefore, point defects occur and display performance deteriorates.